1. Field of the Invention
The present invention relates to a system and method for controlling an engine to facilitate the determination of engine position sensor correction factors.
2. Background Art
The detection of misfires in an internal combustion engine is important for a number of reasons, including fuel economy. When the fuel-air mixture within one or more cylinders fails to ignite, the engine may experience a measurable loss of power. This loss of power, and thus the misfires, can be detected by measuring changes in the velocity of the engine crankshaft. The velocity of the crankshaft can be calculated by measuring the change in the angular position of the crankshaft over a known period of time. The accuracy of the crankshaft velocity, therefore, depends on a number of things, including an accurate measurement of changes in the angular position of the crankshaft.
To measure the angular position of an engine, a variety of devices may be employed. For example, an encoder wheel having teeth or slots may interact with magnetic or optical sensors at predetermined points in the rotation of the engine crankshaft. Inherent in any such measurement device is a certain amount of inaccuracy. In order to accurately determine the engine's angular position, and thus the engine velocity, adjustments must be made to compensate for inaccuracies in the measurements obtained from these devices. In order to determine an appropriate correction factor for a position sensor measurement in an engine, it may be desirable to perform position measurements with engine combustion inhibited. This is because most non-uniformity of engine rotation during normal engine operation is caused by combustion events. A method for “Correction of Systematic Position-Sensing Errors in Internal Combustion Engines” is described in U.S. Pat. No. 5,117,681 issued to Dosdall, et al. on Jun. 2, 1992, which is incorporated herein by reference.
Although many vehicles today are still powered exclusively by an internal combustion engine, hybrid vehicles are becoming increasingly popular. For example, a hybrid electric vehicle (HEV) may include one or more electric motors that can be used to augment an engine to provide power to the vehicle wheels. In such hybrid vehicles, misfire detection, and therefore accurate determination of engine velocity and position, is still important. Therefore, measuring engine position with combustion inhibited—i.e., when fuel is discontinued to the engine—is still desirable for determining correction factors to be applied to the engine position sensor. On a vehicle driven only by an internal combustion engine, a defueled condition occurs during decelerations, which can be initiated by a decrease in driver demand as indicated by the accelerator pedal. On an HEV, however, a decrease in driver demand does not necessarily cause a defueled condition. Therefore, a need exists for controlling an engine in a hybrid electric vehicle to facilitate determination of engine position sensor correction factors.